Flexural behavior and debonding mechanism of RC beams strengthened with FRP mini-bars embedded in geopolymer matrix

Carbon fiber-reinforced polymer (CFRP) offers an efficient strengthening solution, however, pursuing higher load capacities via high reinforcement ratios triggers premature interfacial debonding. Basalt FRP (BFRP) serves as a cost-effective alternative, inherently alleviating interfacial stress concentrations through its lower elastic modulus without sacrificing mechanical performance. This study investigates the flexural behavior and debonding mechanisms of FRP mini-bar-geopolymer systems enhanced RC beams using four-point bending tests and digital image correlation, and 3D finite element analysis. The investigation evaluates the influence of FRP reinforcement stiffness, FRP type, and matrix material. Results indicate that CFRP strengthening utilizing three 4 mm bars (3Φ4) increases the ultimate capacity by 36.6%. Although increasing the reinforcement (6Φ4 or 9Φ4) effectively improves post-yield stiffness, it triggers premature interfacial debonding, yielding no additional ultimate capacity and causing a maximum ductility index loss of 57.7%. Direct BFRP substitution (3Φ4) achieved 82.8% of the CFRP capacity and 64.6% of its post-yield stiffness. The equal-stiffness 4Φ6 design restored comparable performance at 51.1% of the CFRP material cost, but intensified interfacial stress and promoted premature debonding, which was mitigated by the high-toughness matrix. Finally, based on extended parametric simulations, a failure mode map and an empirical boundary equation are established, providing a bidirectional quantitative tool to predict failure modes and reverse-design the allowable strengthening stiffness limit to prevent debonding.